The present invention relates to magnetic tunnel junctions (MTJ), and to the materials used therein. In particular, the present invention provides for the use of ferrimagnetic materials in MRAM (magnetic random access memory) structures and other MTJ structures to significantly improve the operating window in terms of minimum size, coercive field and other related figures of merit.
The memory technology of relevance to this disclosure, named MRAM, is a solid state tunnel junction using magnetic electrodes. For background, reference made to U.S. Pat. Nos. 5,650,958 and 5,640,343 issued to William Joseph Gallagher et al on Jul. 22, 1997 and Jun. 17, 1997, respectively. The storage mechanism relies on the relative orientation of the magnetization of the two electrodes, and on the ability to discern this orientation by electrical means.
In operation as a memory device, an MRAM device can be read by measuring the tunneling resistance (to infer the state of the magnetization of a free or storage layer with respect to a fixed or pinned layer) and written by reversing the free layer magnetization using external magnetic fields. If the free layer is imagined as a simple elemental magnet which is free to rotate but with a strong energetic preference for aligning parallel or antiparallel to the x axis, and if the pinned layer is a similar elemental magnet but frozen in the +x direction, then there are only two relative magnetization states possible for the free and pinned layers of the device: Aligned and anti-aligned.
In current designs for magnetic tunnel junctions (MTJ) used for MRAM sensor or magnetic recording head applications, the device consists of at least three layers: two ferromagnetic electrodes separated by a non-conductive tunnel barrier. The magnetic materials currently used are typical polycrystalline alloys of Ni, Fe and Co. The barrier layer is typically aluminum oxide.
Various parameters are of interest in evaluating the performance of these devices. First, the variation in resistance between the two storage states is described by the magnetoresistance MR-the percentage change in resistance between the two states. Historically ferromagnetic materials with a higher saturation magnetization Ms were used to obtain junctions with higher MR values (see, for example, R. Meservey and P. M. Tedrow, Phys. Rep. 238, 173 (1994)). More recently it has been shown that there is only a weak link between the magnitude of the saturation magnetization and MR of MTJs containing electrodes formed from alloys of Co, Fe and Ni (D. J. Monsma and S. S. P. Parkin, Appl. Phys. Lett. 77, 720 (2000)). Second, the coercive field is of interest since fields generated by currents along wires in the chip will need to be able to rotate the magnetization of the storage layer. As the capacity of memory chips increase, the MTJ area will inevitably become smaller. As this happens, the switching field (also termed the coercive field Hc) rises roughly inversely with lateral dimension, for the same material and thickness and the same aspect ratio and shape of the device. Using current designs, one quickly reaches a situation where junction size dictated by needs for density causes the coercive fields to become unmanageably large.
In addition to these issues, there are other problems that arise when trying to push the size of the devices down into the sub-micron regime. First, there are strong demagnetizing fields which will tend to cause the bit to xe2x80x98erasexe2x80x99 over time. Second, the demagnetizing field is non-uniform: In particular it is strongest close to the edges of the devices, and hence control over the uniformity of the device is most important at the very spot where there is the most difficulty in fabrication. Hence, small defects at the edge of the magnetic element can lead to nucleation or pinning sites for unwanted micromagnetic structures, resulting in unpredictability of the junction properties. Third, using polycrystalline material such as permalloy can lead to increased variation in device properties because of random orientation of the microcrystallites. In very small devices, the statistical fluctuations due to the grain structure will become much more pronounced. These grains also can cause variations in the tunneling properties between electrodes in the MTJ, causing further uncertainty or variation in device properties.
Several directions have been proposed in order to alleviate this rising coercivity problem. First, one may reduce the saturation magnetization of the storage electrode, since Hc scales with Ms. However, many low Ms materials, for example, formed by alloying Co or Fe with non-magnetic elements, give low MR. Second, one may reduce the thickness of the magnetic electrode, since Hc scales with electrode thickness. Current junctions, however, are on the edge of continuity due to their extremely thin electrodes and further reduction is difficult at best. Clearly, there is a need for a new approach to fabricating small devices while still leaving freedom to tune coercive field and providing an appreciable MR.
Briefly, in order to overcome the foregoing problems while attaining the objectives outlined above, the present invention proposes the use of ferrimagnetic materials in magnetic devices, including magnetic tunnel junctions (MTJ""s, which may be used for MRAM) as well as in magnetic sensors such as magnetic heads. Such use of ferrimagnetic materials may be alone or in conjunction with other non-ferrimagnetic materials. Exemplary preferred materials are (Gd,Tb,Dy)xe2x80x94(Fe,Co) alloys.
The present invention therefore broadly provides a magnetic tunnel junction device, comprising: a first layer and a second layer separated by a barrier layer, wherein at least one of the first and second layers comprises a ferrimagnetic material.
In accordance with a preferred embodiment of the invention, one layer of the first and second layers comprises a ferrimagnetic material and the other layer comprises a ferromagnetic material. Desireably, the ferrimagnetic material and the ferromagnetic material each has its magnetization oriented in a plane parallel to said one layer. Alternatively, the ferrimagnetic material and the ferromagnetic material may each have its magnetization oriented in a plane perpendicular to the aforesaid one layer. According to another preferred embodiment, one layer of the first and second layers may further comprise an anti-ferromagnetic material
According to a preferred embodiment, the ferrimagnetic material is characterized by a compensation temperature that exceeds an ambient operating temperature for the aforesaid magnetic tunnel junction (MTJ) device.
By way of example, suitable ferrimagnetic material include at least one of Gd, Tb, and Dy with at least one of Fe and Co, and alloys thereof.
The present invention also provides a memory comprising a plurality of magnetic tunnel junction (MTJ) devices disclosed hereinabove. A memory of this kind is disclosed in U.S. Pat. No. 5,640,343 , supra.
The invention further provides a magnetic sensor comprising a first layer and a second layer separated by a non-magnetic layer, as described hereinabove, where the ferrimagnetic material is characterized by a magnetization oriented in a first orientation, the other layer of the first and second layers comprising a magnetic material which is characterized by a magnetization that is oriented at an angle (preferably 90 degrees) to the aforesaid first orientation.
Preferably, the ferrimagnetic material of the magnetic sensor comprises at least one of Gd, Th, and Dy with at least one of Fe and Co, and alloys thereof, while one layer of the first and second layers further comprises an anti-ferromagnetic material.